Parts of a fossil fish specimen exposed on broken rock surfaces. A portion of the skull is present on the upper right piece, with the tip of the snout being near the right edge of the picture. The team discovered this specimen in 2011. Photo by Ken Angielczyk.
In my two previous posts, I briefly talked about who synapsids are and the need to search for early therapsids in places outside of the traditional areas of study (Russia and southern Africa). This time, let's focus on the area of Brazil we'll be working in, and why we're interested in the rocks that are exposed there. We'll be doing our fieldwork in a geologic structure known as the Parnaíba Basin, which preserves rocks ranging in age from the Cambrian Period (over 488 million years ago) to the Early Cretaceous Period (about 135 million years ago). Today, the basin is located just south of the equator in northeastern Brazil. During the Permian Period (the time in which we're interested), it was located at mid-latitudes in the north-central part of the super-continent Gondwana.
Map of Brazil showing the location of the Parnaíba Basin. Base map by Felipe Menegaz.
The specific rocks that we will be studying are part of a rock unit called the Pedra de Fogo Formation. The name of the formation roughly means “fire stone,” and refers to the fact that certain parts of the unit are rich in chert (or flint), which can be used to generate sparks for starting fires. The rocks of the Pedra de Fogo Formation were formed during the Permian Period of Earth history (299 to 251 million years ago), but currently there is uncertainty about the exact age of the formation. This is because the rocks have only been dated using biostratigraphy, a process in which paleontologists compare the fossils preserved in a rock unit of uncertain age to rock units of known age that preserve similar fossils. Some of the fossils in the Pedra de Fogo Formation are consistent with an age in the early part of the Permian, whereas others are more suggestive of a later age in the period.
The Pedra de Fogo Formation is composed of sedimentary rocks, i.e.,rocks that are formed from sediment generated by the erosion of older rocks, which was transported and redeposited elsewhere. The structure and composition of sedimentary rocks informs us about the environments in which they were deposited. In the case of the Pedra de Fogo, it appears that the rocks were primarily deposited in coastal environments like river deltas and coastal plains, as well as on the floodplains surrounding rivers. Indeed, during the Permian, part of the Parnaíba Basin was likely occupied by a shallow inland sea that was intermittently connected to the ocean to the west, with the rocks of the Pedra de Fogo Formation representing terrestrial environments along the margins of the sea.
Several kinds of fossils have been found in the Pedra de Fogo Formation. These include clams, sharks, bony fish, an archaic amphibian that somewhat resembled a living crocodile, and lots of fossil wood. Indeed, the Tocantins Fossil Trees Natural Monument is located in the Parnaíba Basin, highlighting the petrified forest that is preserved in the Pedra de Fogo Formation. There are also filled-in burrows made by some of the invertebrate animals (likely worms and small crustaceans) that lived in the mud along the shorelines of the bodies of water present at the time.
Dr. Claudia Marsicano stands next to the fossilized trunk of a tree fern in the Parnaíba Basin. Photo by Ken Angielczyk.
Why are we interested in looking for synapsid fossils in the Pedra de Fogo Formation? There are several reasons: 1) The rocks are of the right age. They date from the Permian Period of Earth history, a time when synapsids were some of the dominant animals on land. Moreover, if the formation is from the later parts of the Early Permian, it is roughly the age when we would expect early therapsids to be present. 2) The rocks were formed in the right environment. Rocks that are formed on the floodplains of rivers and in deltaic environments often preserve fossils of terrestrial animals because sediments carried by the rivers can quickly bury animal remains, increasing the probability that they will become fossils. Furthermore, some of the richest synapsid-bearing rocks in North America formed in lowland floodplain and coastal plain environments, much like the environment represented by the Pedra de Fogo. Therefore, the specific environment the rocks sample is one where we know early synapsids liked to live. 3) The previous discovery of fossils in the Pedra de Fogo Formation demonstrates that it is a productive area for finding fossils. In particular, the presence of the amphibian demonstrates that terrestrial vertebrates are preserved in the rocks. 4) The Parnaíba Basin is located in a paleogeographically important place. It's in an area of Gondwana that generally has a very limited fossil record, so any discoveries we make will throw new light on the animals that were living there in the Permian. In addition, its location between the equatorial regions where early synapsids are found and the high latitude areas of southern Africa where therapsids are common puts it along a possible dispersal route that early therapsids could have used to reach southern Africa.
Now all we have to do is find some fossils...